Generally, a ship carries freight such as oil etc. to an importing port, and then the freight is unloaded from the ship. At this time, the ship becomes lighter, and thus the center of gravity of the ship moves upward. For this reason, there are problems in that the ship can be in danger of capsizing because the ship becomes unbalanced, and in that its propulsive efficiency becomes low because its propeller is not fully submerged in water.
In order to solve the above problems, additional tanks, called “ballast tanks”, are installed at the bottom, right and left sides of the body of a ship, and the ballast tanks are charged with water having the same weight as the freight as the freight is unloaded. This water charged in the ballast tanks is referred to as “ballast water”. Ballast water is discharged from the ballast tanks when a ship is loaded with freight.
In other words, the ballast water is charged in the ballast tanks when freight is unloaded from a ship at an importing port, and the ballast water is discharged from the ballast tanks when the ship is loaded with the freight at an exporting port. Since the importing port and exporting port are located in different regions, the region in which the ballast water is charged in the ballast tanks of a ship is different from the region in which the ballast water is discharged from the ballast tanks of the ship. Therefore, microbes, bacteria or the like, living in one region, can be carried to another region through this ballast water.
For example, if red tide plankton exist in the sea near a country, when a ballast tank is charged with ballast water in the sea near that country, the red tide plankton is included in the ballast water. Therefore, when the ballast water is discharged from the ballast tank in the sea near another country, the red tide plankton is also discharged together with the ballast water, and thus the red tide plankton can propagate in the sea near the other country. That is, oceanic creatures living in the sea near a country come into the sea near another country, thus damaging the ocean environment.
For this reason, the International Maritime Organization (IMO) has adopted the “International Convention For the Control and Management of Ship's Ballast Water and Sediments” in the presence of 74 countries in London, England on Feb. 13, 2004. This international convention adopted by IMO provides for ballast water discharge standards. Each country joining this international convention can prohibit the discharge of ballast water if the ballast water discharged from a ship does not meet the ballast water standards. In this case, the ship in question will be unable to load and unload freight.
It is provided in the ballast water discharge standards of this international convention adopted by IMO that the content of microbes such as animal plankton and plant plankton included in ballast water should be 10/100 m3 or less, the content of Cholera bacilli included in ballast water should be 1/100 ml or less, the content of Colon bacilli included in ballast water should be 250/1000 ml or less, and the content of Enterococcus bacilli included in ballast water should be 100/100 ml or less. According to the international convention adopted by IMO, it will be expected that it will be compulsory for all ships to be provided with a ballast water treatment apparatus for meeting the ballast water discharge standards stipulated by IMO by 2017.
Generally, ballast water includes about 0.2˜19‰ chlorine as a main component. In general, ocean water has a chlorine concentration of 19‰, and estuary water has a low chlorine concentration of 0.2˜0.5‰. Here, the unit “‰” is g (chlorine) per kg (water).
Ballast water can be sterilized using a physical process, a chemical process or an electrochemical process. Conventional electrochemical ballast water treatment apparatuses are problematic for several reasons. First, when ballast water is electrolyzed, hydrogen gas is produced together with chlorine, and this hydrogen gas is flammable. It must therefore be diluted to a volume ratio of 1% using external air, and then must be discharged to the outside in consideration of safety problems. For this reason, a high-capacity blower serving to separate hydrogen gas from the electrolyzed ballast water and then supply a large amount of air to the separated hydrogen gas is required, and a large amount of electric power is consumed to operate the high-capacity blower.
Second, the concentration of salt included in sea water used as ballast water is about 3.5 wt %, and the electrolytic efficiency thereof is about 80%. Therefore, about 6.0˜12.0 kW of electric power is consumed to produce 1 kg of chlorine. For this reason, operating costs are increased, and the size of a direct-current power source connected to an electrolytic apparatus is increased.
Third, since conventional processes use a chemical reductant to decrease chlorine concentration to a predetermined concentration or lower when ballast water is discharged, a ship must be loaded with chemicals. This is inconvenient when operating a ship.